Pluto's composition is unknown, but its density indicates that it's
probably a mixture of 80% rock and 10% water ice. Its surface seems
to be covered with ices of methane, nitrogen and carbon dioxide.

Little is known about Pluto's atmosphere, which exists as a gas only when
Pluto is near its perihelion (closest to the Sun). For the majority of
Pluto's longyear, the atmospheric gases are frozen solid. Near perihelion,
it's likely that some of the atmosphere escapes to space, perhaps even
interacting with Pluto's moon, Charon.